Co—Cr—Mo alloys are known for their excellent mechanical properties (hardness, compressive strength) and properties of resistance to corrosion, which have led to a significant diffusion thereof in the biomedical sector, above all in Europe, in particular as material for manufacturing prostheses and dental implants. In said field, the use of said alloys is regulated by the ASTM F75 and ISO 5832 standards.
However, it has been noted that the components obtained by casting Co—Cr—Mo alloys suffer, in general, from microstructural defects linked to the segregation of carbides and to the porosity of the material, which can induce phenomena of localized corrosion and a progressive decay of the mechanical properties.
In order to overcome said drawbacks, alternative manufacturing technologies have been developed, referred to as “rapid manufacturing” technologies, which are based upon additive sintering of powders.
In particular, known, for example from US2006157892, is a method for manufacturing three-dimensional components by means of electron-beam sintering of layers of powders. In addition, known for example from US2009152771 is a method for manufacturing three-dimensional components by laser sintering of powders.
In the aeronautics sector particular interest has been aroused by the possibility of employing Co—Cr—Mo alloys for Manufacturing components for which a high resistance to wear and heat is required, given the typical temperatures of use. However, so far it has not been possible to extend to the applications of the aeronautics sector the technologies based upon additive sintering of powders because the components obtained according to said processes present good hardness and mechanical properties at room temperature, but become particularly brittle when they are exposed to the high temperatures (around 800° C.) typical for the components of aeronautic engines.
In fact, the Co—Cr—Mo alloys that are potentially most promising for these applications contain, in addition to chromium and molybdenum, significant amounts of carbon. The simultaneous presence of these three elements leads to the formation of carbides, which, on the one hand, contribute to bestowing on the material high hardness and considerably good mechanical properties, but, on the other hand, cause embrittlement thereof when they precipitate at the grain boundaries. Precipitation of carbides is thermodynamically favoured precisely in the temperature range of applicational interest in the aeronautics sector.
The main users of the manufacturing systems based upon sintering of powders suggest, in order to overcome this drawback, execution of a heat treatment on the sintered components.
For example, it has been proposed to carry out, on the components obtained by means of laser sintering of powders of Co—Cr—Mo alloys, a stress-relief heat treatment at 1050° C. for two hours, having the chief purpose of minimizing the internal stresses and, hence, the strains of the components (which are undesirable, in particular, in the case of complex geometries).
However, this heat treatment reduces the properties of mechanical resistance, yielding, and ultimate elongation, since, in the course of the process of stress relief, carbides are formed at the grain boundaries. This phenomenon is particularly accentuated in the 700-1000° C. temperature range. A further increase in temperature, for example up to 1050° C., leads to a solubilization of the carbides with a reduction of the embrittling effect caused thereby, but, on the other hand, does not prevent these carbides from re-precipitating in an uncontrolled way in use. In other words, a heat treatment of this sort has proven unadvisable if the aim is to improve the mechanical properties of the components sintered from powders of Co—Cr—Mo alloys.
In addition, it should be emphasized that, in this context, there has not been taken into account the possibility of the components in use being subsequently exposed to high temperatures such as those typical in aeronautics applications.
Alternatively, it has been proposed to subject the components produced by sintering to a treatment of hot isostatic compression (known in the sector also as “HIPping”, from the acronym HIP—Hot Isostatic Pressing) aimed at homogeneizing the material and reducing the brittleness thereof. There is, in fact, obtained a structurally isotropic, recrystallized material without visible carbides within the structure.
However, if, on the one hand, said treatment markedly improves the properties of ultimate elongation of the material, on the other hand, it significantly reduces the mechanical properties thereof as compared to the material as sintered material and significantly increases the production costs.
In addition, said treatment is effective to obtain dissolution of the carbides, but is unable to control the subsequent precipitation thereof when the material is exposed, in use, to high temperatures. In other words, said treatment is useful only for components that find application at relatively low temperatures and, in any case, lower than the temperature range of re-precipitation of the carbides, which is approximately between 700° C. and 1000° C.
The need is hence felt to provide a process for manufacturing a component with a base of Co—Cr—Mo alloys that will enable the drawbacks associated to the solutions known to the art to be overcome.
Furthermore, in particular in the aeronautics sector the need is felt for a process for manufacturing components with a base of Co—Cr—Mo alloys that will enable optimization of the mechanical characteristics at the temperatures of interest for aeronautics applications (up to 800° C.), in particular reducing the brittleness thereof and improving the ductility thereof and the properties of yielding at high temperatures.